1. Field of the Invention
The present invention relates generally to base stations within cellular systems, and specifically to base stations utilizing multi-beam antennas.
2. Background and Objects of the Present Invention
The majority of cellular systems currently allocate the uplink and downlink transmissions in two different frequency bands with simultaneous transmission in both bands. These cellular systems have conventionally included base stations (BS""s) that utilize either a single omni antenna or a single sector antenna (typically 120xc2x0 coverage per sector) for both transmission and reception, for aesthetic reasons and to reduce the costs associated with the antennas, feeders and installation.
To increase the coverage and capacity of systems, adaptive antenna systems using multi-beam antenna arrays have been developed. The narrow beams of the antenna array can be used to increase the sensitivity of the uplink, and to reduce the interference in both the uplink and the downlink. In addition, for diversity purposes, dual polarized antennas are becoming more commonplace, since fading is uncorrelated in orthogonal polarization directions. Traditionally, both polarization directions are used in the uplink, while only one of the polarization directions is selected for the downlink.
A number of proposals for cellular systems employing antenna arrays have been published. Reference is made to the following articles and patents: Forssen et al., xe2x80x9cAdaptive Antenna Array for GSM900/DCS1800,xe2x80x9d Proc. 44th Vehicular Technology Conference, Stockholm, June 1994; Hagerman et al., xe2x80x9cAdaptive Antennas in IS-136 Systems,xe2x80x9d Proc. 48th Vehicular Technology Conference, Ottawa,, May 1998; U.S. Pat. No. 5,515,378 to Roy, III et al.; and PCT International Application WO 95/34102 to Forssen et al., each of which is hereby incorporated by reference.
Reference is now made to FIGS. 1 and 2 of the drawings, which show a block diagram of a conventional multi-beam antenna array 50 and a typical beam configuration from a conventional multi-beam antenna array 50, respectively. As shown in FIG. 1, present multi-beam antenna array 50 configurations typically include at least one antenna aperture 55 consisting of several rows and columns of radiating elements. The antenna array 50 generates a plurality of narrow azimuth beams or lobes (shown in FIG. 2) via beamports using the full array 50, where the direction and shape of the antenna beams (Beams 1, 2, 3 and 4) are determined by one or more beamforming networks 46 connected to the columns of the array 50. The beamforming networks 46a and 46b utilizes signal amplitude and phase relations between different columns of the array 50 to create the beams.
Typically, the multi-beam antenna arrays 50 have one duplex filter 70 connected to each antenna beam or one duplex filter per antenna column. Each duplex filter 70 is responsible for selecting the appropriate signal for transmission on the downlink and for selecting the appropriate signal for reception on the uplink. Normally, there will also be an additional selective filter 60 in front of each low noise amplifier (LNA) 40 in the uplink.
For example, as shown in FIGS. 1 and 2, if the two orthogonal polarization directions are approximately linear polarized slanted xc2x145xc2x0, with the downlink using only one of the polarization directions (here +45xc2x0) and the uplink using both polarization directions (+45xc2x0), the beamforming network 46a that produces the +45xc2x0 polarization direction includes duplex filters 70 to handle both the uplink and downlink signal selections, while the beamforming network 46b that produces the xe2x88x9245xc2x0 polarization direction includes only selective filters 60 to handle the downlink signal selections.
Unfortunately, the duplex filters 70 in current systems introduce losses to the transmitted and received signals in the system. These losses result in smaller coverage areas for the base station""s implementing multi-beam antenna configurations. To make these losses manageable, and therefore to increase the coverage area of the base station""s, large cavity filters are typically used. However, large cavity filters are expensive and require a large amount of volume, especially for multi-beam arrays where several parallel filters are required, one for each beam.
The possible alternative of using two different antennas, one for the uplink and one for the downlink, will only add additional cost for the feeder and antenna hardware as well as for installation. In addition, two antennas are often impossible to implement due to aesthetic reasons.
It is, therefore, an object of the present invention to provide a multi-beam antenna configuration without utilizing duplex filters.
The present invention is directed to cellular systems and methods for providing a multi-beam antenna within a base station without utilizing duplex filters. The multi-beam antenna of the present invention produces overlaid beams in both the uplink and downlink to cover the same area. Each beam has orthogonally oriented polarization directions (e.g., linear polarized slanted xc2x145xc2x0) for the uplink and downlink. Adjacent uplink beams also have shifted polarization directions in order to attain polarization diversity between different beams. In order to implement this polarization configuration, for each beam, there is a selective filter in front of the low noise amplifier for the uplink, and no duplex filter in front of the amplifier in the downlink direction. In the simplest form, this solution is also applicable to antennas with broad sector-coverage beams, where one polarization is used in uplink and the other in downlink. Advantageously, by removing the duplex filters from the multi-beam antenna, the downlink losses are reduced. Similarly, since the total amount of filters are reduced, the total cost and volume required for the antenna system is reduced.